Documentation: explain [KMG] parameter suffix

The '[KMG]' suffix is commonly described after a number of kernel
parameter values documentation.  Explicitly state its semantics.

Signed-off-by: Ahmed S. Darwish <darwish.07@gmail.com>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>